(CNN) -- Robert Winn met his wife, Christine, in college. He was a fraternity boy. She was a sorority girl. Early in their relationship, he made a confession, a thorny secret he camouflaged from his closest family and friends.

The truth sputtered out awkwardly.

Sensing his nervousness, she speculated he would announce he was sick -- or perhaps dying?

He told her he was bisexual.

On the surface, Robert Winn, now 40, and Christine Winn, 41, appear to be like any other blissfully married heterosexual couple. They boast nearly 18 years of monogamous marriage. He's a well-respected physician, who works with the LGBT community in Philadelphia, Pennsylvania. She's a successful hospital administrator.

The couple says they've grown closer over time, but like any marriage, two people can have differences -- including sexual orientation. Christine Winn is straight, and she has been supportive of her husband, who is openly bisexual.

"I don't think about it [his bisexuality] as a part I have to accept," she said. "It's just a part of him like any other husband who loses their socks on the floor or doesn't take the trash out."

Her husband feels a sexual and emotional attraction toward men and women. While he fantasizes about Angelina Jolie just as his straight male friends might do, he is also attracted to Brad Pitt.

This may sound like the best of both worlds, but being openly bisexual can be complicated. He frequently battles the stereotypes of bisexuality: That bisexual men are promiscuous. That his relationships with men were just an adolescent phase. That his bisexuality is imaginary. That he's really a gay man trying to camouflage his orientation.

"There is a whole list of assumptions of what my life might be like, that somehow she is some sort of front for me because I'm not willing to accept I'm gay," he said. "People are confused by bisexuality. There's just not a lot of support for people who fall in the middle like me."

More than 50 percent of Americans accept the idea of a gay or lesbian relationship, signaling growing support for same-sex couples, according to a Gallup poll in May. The poll, however, doesn't address the issue of bisexuality, often defined as having a romantic attraction to both men and women. It's a sexual orientation some advocacy groups and researchers say remains challenging because neither the gay community nor the straight population advocates for men and women who are attracted to both sexes.

"It's either you're in the closet or out of the closet, and it's not that simple," David Malebranche, a physician and professor of medicine at Emory University, says about the common perception of bisexuals.

About 1.8 percent of men and women between the ages of 15 and 44 identified themselves as bisexual, according to a 2005 survey from the Centers for Disease Control and Prevention. Researchers caution the government's figures are murky because many bisexuals will not identify themselves.

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Ben Pierce, a 22-year-old recent college graduate living in Massachusetts who identifies as bisexual, can understand why bisexuals are hesitant to come forward. He likens being bisexual today to being biracial in the 1960s, a period when racism and discrimination were widespread. A person who was mixed race often couldn't feel comfortable among either racial group, Pierce explained.

"You're caught in between these two very different groups of straight people and gay people, and neither one really accepts you," he said.

Over the last century, scholars began to examine the dual attraction to females and males through scientific research. In the 1920s, psychologist Sigmund Freud theorized that bisexuality was an innate trait found in humans. Several decades later, sex researcher Alfred Kinsey became infamous for his six-point scale to determine whether a person was heterosexual, homosexual or in between. The spectrum showed there are varying degrees of homosexuality and heterosexuality.

Some sociologists say bisexuality has become more tolerable among the mainstream world, particularly younger generations. The word "fluid" is often tossed casually across college campuses to describe the mixed feelings for both genders. Sociologist Eric Anderson is examining college student perceptions of bisexuality in the U.S. and England. So far, his results among college athletes showed that 90 percent of the students surveyed believe bisexuality exists.

"There is more exploration of sexuality -- even visible dating and exploring -- versus before, when it was more closeted," said Shane Windmeyer, director of Campus Pride, the leading national organization for LGBT college students.

Some say that coming out as bisexual has been easier for women than men. In recent years, several Hollywood female stars have proudly declared their bisexuality. Female celebrities like Lady Gaga, Lindsay Lohan and HBO "True Blood" actress Anna Paquin have said they are bisexual.

"It's [female bisexuality] something that's tolerated because sometimes men see it as entertaining and exciting for them," said Denise Penn, director of the American Institute of Bisexuality.

That hasn't been the case for bisexual men like Robert Winn, the Philadelphia physician now married to a woman.

Coming out bisexual in the 1980s was an agonizing experience for Winn, who was raised Methodist in a military family. His childhood was filled with feelings for girls and boys. He dated his first girlfriend at the age of 15, a teenage romance that eventually ended.

Months later, strong feelings erupted for someone else: a boy at his high school. The two boys shamefully kept their relationship underground.

"Afterwards, we would be embarrassed, confused and not talk about it and then go on with our lives in a normal context," Winn said.

When Winn was a teenager in the 1980s, public support toward gays and bisexuals plummeted as the HIV panic stigmatized the gay community. Bisexuals were blamed for spreading the virus to the straight population, experts said.

Winn realized then there was an unexpected upshot of bisexuality.

It's [female bisexuality] something that's tolerated because sometimes men see it as entertaining and exciting for them.--Denise Penn, American Institute of Bisexuality

"I always had this heterosexual relationship to fall back on," he said. "I could choose to ignore the rest and put it on the back shelf."

Joshua Verbeke, a 29-year-old business student at Indiana University, knows about masking his bisexuality. Verbeke recalls working for a gay advocacy organization that was trying to eliminate sexual orientation discrimination in the workplace. There were instances, Verbeke admitted, when he played along with being gay to avoid criticism and questions about being bisexual. He's heard the phrase "Bi now, gay later" many times.

"I think a lot of bi people are afraid to come out because there is pressure on both sides," said Verbeke, who is dating a 33-year-old man who identifies as gay.

Tensions exist between the bisexual and gay communities, say advocacy experts. Take, for example, the recent legal debacle over a softball tournament. Earlier this spring, three bisexual softball players sued a softball league for stripping their team of a second-place finish at the Gay Softball World Series in 2008.

The suit, filed in the U.S. District Court in Seattle, Washington, alleges that the softball league had discriminated against the three bisexual men by removing the second-place title from the team for being "non-gay."

The academic world has also questioned the idea of bisexuality. In 2005, a controversial study from professors in Toronto, Canada, and Illinois reported males identifying as bisexual were typically not aroused by both sexes. Most of the bisexual men surveyed were physically aroused by images of men instead of women, the study said.

The bisexual -- and gay -- community lashed out against the study, but the study did spur more research on bisexuality. Most of the research on men who have sex with men over the last decade has been driven by the startlingly increasing rates of HIV among the black population. Now, some of the research is changing.

Brian Dodge, a research scientist of public health at the Center for Sexual Health Promotion at Indiana University, is researching sexual health among bisexual men. The American Institute of Bisexuality is considering studying the cognitive behavior of bisexual men through brain scans.

Despite the skepticism from others, some bisexuals acknowledge there are upsides to their sexual orientation, even though they remain in limbo between two worlds. John, 41, a bisexual from California, said his sexual orientation makes him open-minded. For privacy reasons, he declined to give his last name.

"The world is not black or white to me, but a rich diversity of colors, and it is not either/or, but both/and," he wrote in an e-mail.

I think a lot of bi people are afraid to come out because there is pressure on both sides.--Joshua Verbeke, 29-year-old bisexual

What other people think is mattering less to Robert and Christine Winn as they get older and their bonds grow deeper. In the earlier years of their marriage, he said his wife felt ostracized by the gay community when she attended his medical fundraisers and conferences. The gay patients and colleagues were often surprised he was married to a woman. Nevertheless, she supported him.

Could married life have been easier if Robert stayed quiet about his bisexuality? Probably so, he says. He ponders that sometimes.

But then Robert would be lying to himself about his identity. And that's something he never wanted to do to himself -- or his wife.

"I didn't want to turn 40, and then come out to my wife about it," he said. "I wanted to be open with her about who I am and what I think. Thankfully, she was willing to accept me."